The present invention relates to a balanced transconductor suitable for use in active electronic filters and an electronic device comprising a balanced transconductor.
In the field of wireless communications, a preferred radio receiver architecture for achieving a high sensitivity fully integrated wireless transceiver is the low-IF (intermediate frequency) polyphase receiver architecture. One of the keys to the success of this polyphase architecture is the ability to integrate the channel filter. In order to increase the level of circuit integration and to reduce power consumption, it is desirable to implement wireless transceivers in mixed analogue and digital integrated circuits (ICs) and to implement such mixed signal ICs in a low voltage digital CMOS process.
One component which can be used to implement an integrated channel filter is the transconductor. Transconductors are important components for many types of electronic circuit. They form the basis of the transconductor-capacitor (Gm-C) class of active filters, and with extra switches they create switched-current memories for sampled-data filters. Ideally, they linearly convert an input voltage into an output current with both input and output ports presenting infinite impedance.
Therefore, it is desirable to devise transconductor circuits that have a high performance and can be implemented in a low voltage digital CMOS process.
A single-ended transconductor cell illustrated in FIGS. 1 (circuit schematic) and 2 (block schematic) employs a PMOS/NMOS transistor pair. If the PMOS and NMOS transistors are dimensioned to have equal transconductances gm, the overall transconductance of this single-ended cell is xe2x88x92G=xe2x88x922.gm. Further, if they have equal threshold voltages, and using a pair of voltage supply rails Vss and Vdda, for simplicity assuming Vss=0, and biasing the input voltage at the mid-rail voltage, Vdda/2, equal drain currents J are produced in both the PMOS and NMOS transistors and the output current iout is zero. When the input voltage changes by vin, the drain currents of both transistors are unbalanced and a linearly related current, iout=xe2x88x92G.vin, flows at the output. The transconductor is very efficient because it operates in class AB and clipping does not occur until the output current iout reaches 4.J.
A balanced transconductor, which converts a balanced input voltage into a balanced output current, is illustrated in FIGS. 3 (circuit schematic) and 4 (block schematic). It comprises two of the single-ended transconductor cells of FIGS. 1 and 2, one converting the positive signal voltage and the other converting the negative signal voltage, and it achieves a differential transconductance Gd=G/2. In active filters, inversion of signals is frequently required and can be achieved by simply crossing over signal pairs. Unfortunately, if this simple balanced transconductor is used in feedback networks such as those occurring in active filters the circuits become unstable. Use with feedback networks is illustrated in FIG. 5, and equivalently in FIG. 6 in which a negative feedback loop comprising inverting and non-inverting single-ended transconductors (FIG. 5) is implemented with all inverting single-ended transconductors with wired cross-overs creating the extra inversion (FIG. 6). In this configuration the four inverting single-ended transconductors form a positive feedback loop and in practice the circuit behaves like a digital latch with the inputs and outputs switching to one or other of the supply rails.
The problems of instability can be resolved by using a common-mode feedback network having four additional single-ended transconductors configured as described in xe2x80x9cLinear CMOS transconductace element for VHF filtersxe2x80x9d, B. Nauta and E. Seevinck, Electronics Letters, 30th March 1989, Vol. 25, No. 7, as shown in FIG. 7. Assuming that the PMOS and NMOS transistors have identical parameters, then the common-mode input voltage producing zero output current is vin+=vinxe2x88x92=Vdda/2. With a purely differential input signal voltage vdm, i.e. vin+=Vdda/2+vdm/2 and vinxe2x88x92=Vdda/2xe2x88x92vvm/2, the common-mode feedback network produces currents which cancel and the network between the two inputs presents infinite impedance. With a purely common-mode input signal voltage vcm, i.e. vin+=vinxe2x88x92=Vdda/2+vcm, the common-mode feedback network produces currents which add and the network produces resistive loads at the outputs. The differential and common-mode equivalent block schematic diagrams are shown in FIGS. 8 and 9 respectively.
A feedback loop containing two of the balanced transconductors of FIG. 7 is stable because the common-mode voltage gain of each of the additional single-ended transconductors is 0.5 giving a loop gain of 0.25. However, loops containing both this and the simple balanced transconductor of FIG. 3 are unstable. Filters made exclusively from the balanced transconductor of FIG. 7 will consume three times the power of a filter (albeit unstable) made from the simple balanced transconductor of FIG. 3 and this is a heavy penalty. Furthermore, the balanced transconductor of FIG. 7 has a common-mode rejection of only 0.5 (6 dB).
An object if the present invention is to provide an improved balanced transconductor and electronic device comprising a balanced transconductor.
According to one aspect of the invention there is provided a balanced transconductor comprising a first signal path between a first input terminal and a first output terminal, a second signal path between a second input terminal and a second output terminal, the first signal path comprising a first single-ended transconductor means and the second signal path comprising a second single-ended transconductor means, and a cancellation network coupled to the first and second signal paths, wherein the cancellation network is configured to provide cancellation at input ports of the first and second single-ended transconductor means of a common-mode voltage applied to the first and second input terminals.
By providing cancellation of a common-mode voltage an improved common-mode rejection ratio is obtained, and improved stability is obtained in feedback loops made using either two of the balanced transconductors according to the invention or even from one such balanced transconductor and a pair of single-ended transconductors.
Preferably the cancellation network comprises half-size single-ended transconductor means which use half-width transistors and draw half the supply current of the first and second single-ended transconductor means, thereby contributing to a low power consumption. Alternatively, other size transistors may be used. Small transistors will draw small currents.
According to a second aspect of the invention there is provided an electronic device comprising a balanced transconductor in accordance with the first aspect of the invention.